13.734 - Sailing Yacht Design
Fall 2003
Due September 24, 2003
Problem Set 2
Sailing Dinghy Design
This problem centers on the design of a sailing dinghy. The work should be done with
MaxSurf
1. Generate a “starting point” of the sailing dinghy by using Maxsurf to scale the
length, width and depth of the design you did in the getting started exercise in class.
Scale the length to 3.5 meters, the total breadth to 1.6 meters and the total Depth
(draft plus freeboard) to 0.9 meters. If you would like to have a little less freeboard
for your own reasons, you can scale the depth to 0.7 or 0.8 meters.
2. Make sure the control point net has four rows and five columns. Except for the
column at the bow, all other columns should have their control points in a plane.
Adjust the control point positions slightly to achieve this if necessary. If your
“getting started” design has only four columns add a fifth column so there are
columns along the length at approximately 0, 25, 50, 75 and 100% of the length.
3. Run the upright hydrostatics and you should find that the displaced weight is
different than the desired weight of 300 kg. Adjust the three lowest control points
in each column to change the displacement and get a shape which pleases you and
has a displaced weight very close to 300 kg in seawater with a density of 1025
Kg/m3. You can also move the DWL up or down to alter the displacement. Make
sure that the fore and aft perpendiculars are set at the final bow and stern endings of
the design waterline. When you run the upright hydrostatics you will see, amongst
other things, the LCB and the prismatic coefficient. Your design values should
have a Cp of about 0.6 and an LCB about 0.16 meters aft of amidships.
4. Make a grid with 11 equally spaced sections including the aftmost and forwardmost
sections. Set five waterlines located at z = -0.2, 0.0, 0.2, 0.40 and 0.6 meters. Set
four buttocks located at y = 0.0, 0.2, 0.4, and 0.6 meters.
5. Make printed graphs of your sections, waterlines and buttocks.
6. Make and print the upright hydrostatics data from Maxsurf. Use center of gravity
heights of 0.5 m for the 175 kg crew, 4 m for the 25 kg sail and rig and 0.1 m for
the 100 kg hull.
7. Design a centerboard and rudder for the dinghy. The centerboard should have a
span of about 1 meter, and a mean chord of about 0.4 meters. The rudder should
have a span (below the DWL) of about 0.8 meters and a mean chord of about 0.3
meters. Thickness fractions of 9% to 11% are reasonable values.
8. The sailing stability of the boat, at about 10 degrees of heel, a typical upwind
sailing angle, is 10 times the righting moment per degree and 175 x 0.8 kg-m from
the two person crew on the rail. Design a catboat (single almost triangular sail)
sailplan which will give about the same amount of heeling moment at an apparent
wind speed of 17 knots (8.75 m/s). The density of air is about 1.2 kg/m3, depending
on temperature. Use a mean lift coefficient of 1.2. Base the heeling moment on
presuming that the center of aerodynamic side force is 40% of the sail height above
the bottom of the sail. At this height, the center of force is about 35% of the sail
chord aft of the leading edge. Approximate the side force as the lift force. When we
later do sail aerodynamics, you will learn how much drag is associated with a
prescribed lift so you will be able to calculate the side force for exactly for a given
apparent wind angle.
9. Find the center of lateral resistance of the boat by approximating the side force on
the hull and centerboard as the force on a centerboard alone, but extended up to the
waterline with is center ¼ of the chord length aft of the leading edge at a height that
is 40% of the total draft below the waterline, and a force on the rudder that is 20%
of the force on the keel and hull and located ¼ of its chord length aft of the leading
edge at a depth of 40% of the rudder span.
10. From the above information, determine the fore and aft location of the mast such
that the longitudinal positions of the aerodynamic side force and the hydrodynamic
side force coincide.
11. Run large angle stability analysis on the boat using Hydromax. Find the angle of
heel for maximum GZ with the crew on the rail and with the crew on centerline.